In the well known art of electrostatography a latent electrostatic image formed on the surface of a recording member is rendered visible or toned by application thereto of electroscopic marking particles which may be in the form of dry powder or in liquid dispersed form known as liquid toner. The recording member may comprise a photoconductive or dielectric printing element or an electrostatic master having insulative charge accepting and conductive non-chargeable areas and such like materials.
There are many prior art methods of applying liquid toner to such recording members. In one of such prior art methods it has been proposed to tone a recording member in the following manner. The recording member is held by suitable means against a flat platen, image bearing surface down. The platen is passed over a relatively short so-called developing electrode as is well known in the art and toning of the latent image on the recording member surface is effected over a narrow zone formed between such surface and the developing electrode where such zone is filled with liquid toner. As the platen with recording member traverses the electrode, the toning zone is constantly filled with fresh toner. This is attained by the following means. As the platen with the recording member commences traversing the electrode in the forward direction, it passes first over a supply roller which precedes the electrode and is partly immersed in toner contained within a reservoir located beneath said roller. The top of such supply roller is level with or is very slightly below the electrode surface. The edge of the electrode nearest the roller is in the shape of a knife-edge placed very close to the roller or nearly in contact or in virtual contact therewith. The supply roller rotates in the same direction as the platen traverses the electrode and the toner carried on the roller surface is removed therefrom by the electrode knife-edge and directed over the electrode surface, that is to say into the toning zone formed between said electrode surface and the traversing recording member surface. The speed of rotation of the supply roller is adjusted so that the volume of liquid carried by the roller to the electrode surface is sufficient to keep the toning zone full with fresh toner as the recording member traverses thereabove at some given speed.
It will be realised that this prior art toning mechanism is effective only when the platen is traversing the electrode in the forward direction because if its direction of traverse is reversed, the toning zone will not be filled with fresh toner as the direction in which the roller supplies toner to the electrode, that is the direction in which the toning zone is filled with toner will be opposite to the reverse movement of the platen. Thus it will be seen that there is need for a toning mechanism where the toning zone is constantly filled with fresh toner during the forward as well as the reverse traverse of the platen to obtain efficient toning in both directions.